I know there are a few causes of this error however in my instance it was due to Microsoft Exchange Replication Service not started. The service had been misconfigured to run as a service account with invalid credentials.

Setting it back to local system resolved the issue and getting the customer back online.

The Active Sync service queries "Active Manager" which is part of the Exchange Replication Service to determine where the users active mailbox copy in a DAG resides - and still follows this model even if your not using DAG's in your environment. Hence if Active Manager is not available, no one can locate their Active Mailbox.